In a positive crankcase ventilation system of an internal combustion engine for circulating crankcase vapors into an intake system to prevent them from being discharged directly into the atmosphere, a blow-by gas passage, communicating a crankcase and a rocker arm chamber of the internal combustion engine with each other, and an oil return passage, are generally formed as one and the same. In a V-type internal combustion engine, which has left and right cylinder banks arranged offset in a lengthwise direction in which a crankshaft of the V-type internal combustion engine extends, it is preferable to use part of each cylinder bank left as a dead part or corner on one end of a row of cylinders formed in the same cylinder bank to form a blow-by gas passage in the dead corner independently of an oil return passage. This not only brings about an improved efficiency in returning oil, but decreases contamination of oil into blow-by gases. Such a formation of oil return and blow-by gas passages is known from Japanese Unexamined Patent Publication No. 62 170,715 entitled "V-Type Engine", published July 27, 1987.
It is a well known fact that, because main bearings of a V-type internal combustion engine are loaded diagonally downward from the right and left, with tremendously heavy loads produced as a result of fuel explosions and inertial forces of pistons of the internal combustion engine through a crankshaft, the V-type internal combustion engine must have an inadequate dynamic stiffness of a lower structure of a cylinder block, such including main bearings and main bearings caps.
To give such a sufficient stiffness, it is known to provide reinforcing bulk heads integrally formed with and extending downward from the main bearing caps of the main bearings rotatably holding the crankshaft of the V-type internal combustion engine and to connect them integrally with one another. In such a lower cylinder block structure, however, a crankcase is unavoidably partitioned into several small crankcase chambers, resulting in insufficient crankcase ventilation. Even though the blow-by gas passage can be formed on one end of each row of cylinders in the dead corner of each cylinder bank of a V-type internal combustion engine, the small crankcase chambers partitioned by the reinforcing bulk heads worsens the flow of blow-by gases and, therefore, are still hard to sufficiently ventilate. This leads to a gradual deterioration of oil quality, due to contact of the oil with blow-by gases staying in the crankcase. When an air introduction passage for positive crankcase ventilation is provided or formed on one end of the row of cylinders in the dead corner of each cylinder bank, the air introduction passage is apt to be filled with, oil in the crankcase due to the oil getting rough or agitated when a vehicle, starts or turns, so as to cause ineffficient operation of the positive crankcase ventilation system of the V-type internal combustion engine.